Update cli message with correct command

[r-cha]

With a fresh project, using SaaS template with frontend assets, the CLI instructs the user:

Next step, build your frontend:
  $ cd frontend/
  $ npm install && npm build

but this causes

Unknown command: "build"


Did you mean this?
  npm run build # run the "build" package script
To see a list of supported npm commands, run:
  npm help

The obvious fix is to print the proper message, npm run build.